Yeast, when examined under the microscope, is found to consist of globular vesicles about 1/2300th part of an inch in diameter when fully grown. They are multiplied by little vesicles budding out from the sides of the parent. These soon acquire an equal size, and repeat the reproduction, either while attracted to the parent globule or after separating from it. The multiplication goes on to an indefinite extent with a fitting supply of food and at a moderately warm temperature (70 deg.-90 deg. Fahr.). The vesicles are nourished by sucking in a portion of the organic liquid in which they exist, decomposing this chemically, and either actually giving off, or causing the separation of their outer surface, of carbonic acid in the form of gas. To give a familiar illustration of the action of the carbonic acid which is evolved from yeast on the dough, I may say that it is analogous to the froth formed on a tumbler of bottled ale or ginger-beer. The cavities or bubbles in the dough are produced in an exactly similar manner; but two circumstances occur in bread to render them permanent—first, the fact that they are slowly formed; secondly, that they are generated in a substance which, while it is soft enough to allow the bubbles to expand, is tough enough to retain them. There are several kinds of yeast besides barm, or brewer’s yeast, which, in spite of its bitter taste, is generally used by bakers because it is the least expensive. Next in consumption is what is termed press yeast, in German _press hefe_ or _pfund hefe_, commonly known in commerce as German yeast, so called because it originally was a monopoly of that country, but it is now largely manufactured in Scotland. Of these yeasts Mr. Austin says: ‘Press yeast is obtained partly by the brewing of beer or distillation of spirits as a by-product, partly it is made artificially. In the former case, the beer upper yeast is mixed with ten times its quantity of water, to which one per cent. of carbonate of ammonia is added, macerated and well washed for an hour, and then mixed with a compound of two parts of finely-powdered malt and ten parts starch, so that we have a firm mass, which is made into cakes half-an-inch thick. This yeast must be made fresh every two or three days, and must be kept in a cool place. A better press yeast is made from the yeast of the distilleries. The pasty residue of the mash tub is passed through a hair sieve to get rid of the grain husks. The filtrate is allowed to settle, and the sediment is put into linen cloths and washed with water, and the water squeezed out again under gentle pressure. The yeast is thus obtained in the form of cakes.’